Electrostatic discharge (ESD) is generated due to mutual contact or separation from each other of at least two articles. These kinds of phenomenon, i.e., mutual contact and separation from each other, are referred to as triboelectic charging, which is related to a rapid charge transfer between different articles respectively having different surface potential level. The so generated charge is then remained on the surface of the articles to lead to ESD. The charge imbalance of articles will create an electrostatic field. Furthermore, the electrostatic field will continue to accumulate and then cause the electrical charge transfer of articles due to the different surface potential. This phenomenon is known as ESD which is one of the main reasons for device failure in the semiconductor industry. The ESD damage for the semiconductor industry surveyed by the American National Standards Institute (ANSI) shows that, the ESD damage leads to an average loss of product in the range between 8% and 33%.
An electronic device will be permanently damaged when high-voltage electrostatic passes therethrough. The failure of electronic device caused by ESD includes electricity leakage, short circuit, and brightness down or burned.
Machine Model (MM) refers to an electrostatic discharge from a charged machine. The charged machine contacts an electronic device and discharges electrostatic from the electronic device to lead to the failure of the electronic device. The static electricity from MM is generally eliminated by a grounding process.
Human Body Model (HBM) is another common type of ESD that a charged human body directly transfers its accumulated electrostatic charge to an electronic device by touching it, and which is also most difficult for being completely eliminated. Electrostatic accumulated in the human body is up to 3 kV, and electrostatic accumulated in clothing is up to 8 kV in a dry environment. When a human with a positive potential or a negative potential touches one electronic device, the accumulated electrostatic is discharged and flows into the electronic device to pass a very high voltage or current to the electronic device to lead to a serious irreversible damage to the electronic device. Therefore, the staffs who have chance to touch the electronic device in a factory must be equipped with anti-static equipment for safety of device.
In the practice, LEDs are tested and classified, referred as sorted in the present invention, with regard to electrostatic resistance by using random sampling method. First, an electrostatic gun, handheld by the staff, discharges an electrostatic to LEDs. Then, the LEDs characteristics of brightness, electrical resistance value and so on are detected. Finally, the LEDs failure rate is estimated according to the random sampling ratio. The estimated LEDs failure rate is approximate to the LEDs real failure rate when a high percentage of random sampling ratio is achieved. When a high estimated failure rate for the LEDs is obtained, we can predict that the most of these LEDs have a weak electrostatic resistance for the testing voltage value. Conversely, when a low estimated failure rate for the LEDs is obtained, we can predict that the most of the LEDs have a strong electrostatic resistance for the testing voltage value.